This invention generally relates to propeller de-icing systems.
Aircraft propellers are typically deiced by heating the accreting surface of the propeller blades. Electrical resistance heaters typically provide the heat. The heaters raise the surface temperature of the blade (when the blade is covered by some amount of ice) until the bond between the ice and the blade surface is significantly weakened. The forces associated with the propeller rotation shears the bond between the ice and the propeller blade such that the ice is removed.
It is important in propeller de-icing systems to not melt the ice to avoid water runback onto portions of the blade that are not protected by heaters. Conventional arrangements suffer from the drawback that they tend to melt ice, which introduces the possibility for water runback onto the unprotected portions of the propeller blades.
In order to avoid melting ice with conventional arrangements, heater power levels are minimized. In very cold conditions, sufficient ice must built up to insulate the heater surface before the temperature at the interface between the propeller blade and the ice increases sufficiently to weaken the bond and remove the ice. Anticipated propeller performance under these icing conditions is reduced because of the relatively significant build-up of ice.
A significant drawback to conventional arrangements is that they typically merely control heater on and off times. This provides constant power for constant input voltage because conventional arrangements use heaters having resistance that is stable with variations in temperature. Energy delivered to the heaters is roughly proportional to the time that power is applied. Conventional methods of propeller de-icing turn on power to a blade heater for a controlled time. For low ambient temperatures, the time is generally longer than for warmer ambient temperatures.
There is a need for an improved propeller de-icing system that improves propeller performance by reducing the amount of ice build-up and, moreover, avoids the possibility for water runback onto portions of the propeller blade that are not heated. This invention addresses that need while avoiding the shortcomings and drawbacks of the prior art.
In general terms, this invention is a propeller de-icing system that utilizes ambient temperature information and resistance information regarding the heater to control heater operation.
One example system designed according to this invention includes a propeller having a plurality of blades. Heaters are associated with the blades. The heaters have a coefficient of resistance that provides a predictable relationship between heater temperature and the resistance of the heater. An electronic control unit determines a desired heater temperature, based upon the ambient temperature. The controller then powers the heater and monitors the heater resistance until the resistance corresponds to the desired heater temperature.
The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows.